Semiconducting metal oxide surfaces

Other inorganic reactions shown to be photo-induced at colloidal semiconducting metal oxide surfaces include the synthesis of ammonia from water and nitrogen (19) and the oxidation of halide ions 1 ,... 

The sensing action by metal oxides depends on several factors such as grain size (available surface area) and surface states as well as the efficiency with which the test gas molecules adsorb on the surface [25-27]. The sensing mechanism of n-type semiconducting metal oxides involves... 

Both HREELS and RAIRS have been applied extensively of the study of adsorbates on metal surfaces. The extension of the techniques to semiconducting or insulating oxide surfaces is hampered by a number of problems. The result is that until even relatively recently [9] there were only a couple of examples of RAIRS studies on oxides, and these were confined to polycrystalline systems. Most early HREELS studies were concerned with the characterisation of the intrinsic phonon modes of metal oxide surfaces. This contrasts strongly with the extensive literature concerning the vibrational characterisation of adsorbates and intermediates on powdered oxide surfaces that have been obtained by transmission or diffuse reflection IR techniques. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

The depth of this space-charge layer (L) is a function of the surface coverage of oxygen adsorbates and intrinsic electron concentration in the bulk. Before the sensor is exposed to reducing gases, the resistance of an n-type semiconducting metal oxide... 

Various types of solid-state NO2 sensors have been proposed based on semiconducting metal oxides (including heterocontact materials) [42-50,58,59,234-238], solid electrolytes [1,239,240], metal phthalocyanine [241], and SAW devices [242]. Among these NO2 sensors, the semiconducting metal oxides and solid electrolytes appear to be the best. Specifically, semiconducting metal oxide gas sensors are most attractive because they are compact, sensitive, of low cost, and have low-power consumption. Their basic mechanism is that the NO2 gas is adsorbed on the surface of the material this decreases the free electron density into the space-charge layer and results in a resistance increase [243]. 

Fig. 15.4 Principle of the optical gas-sensing effect, (a) Schematic iiiustration of goid nanoparticles embedded in the volume and on the surface of a semiconducting metal oxide layer with refractive index n. (b) Shift of the absorption peak of a single gold nanocluster (75 nm in diameter) by a variation of the refractive index of the surrounding medium at exposure to a reducing or oxidizing gas (Reprinted with permission from Schleunitz et ai. 2007, Copyright 2007...

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